1. Field of the Invention
Embodiments of the invention generally relate to integrated circuit optical waveguides.
2. Description of the Related Art
As the demand for more powerful microprocessors increases, the interconnection technology within integrated circuits (ICs) must also advance in order to support the next generation of faster and more powerful microprocessors. Conventional ICs utilize electrical signals for data transmission. However, since an optical signal propagates at a velocity that is faster than the propagation velocity of an electrical signal, optical transmission mediums and interconnect devices have an inherent ability to transmit data at higher speeds. Therefore, optical signals offer great potential for increasing the signal transmission rate within ICs. Furthermore, several optical signals may share a single common optical transmission medium without interfering with each other. Alternatively, when more than one electrical signal occupies a transmission medium, interference often occurs. This interference, generally termed crosstalk, is defined as a disturbance caused by the electric and/or magnetic fields of a first signal affecting the electric and/or magnetic fields of a second signal in the same or an adjacent transmission medium. As a result of crosstalk, the signal transmission density is substantially greater for optical signal based systems than for electrical based systems. As a result of these differences, along with other advantages of optical based systems, optical signal transmission methods and processes are an attractive option for supporting the next generation of faster and more powerful ICs and microprocessors.
Current technology generally supports optical signal transmission through, for example, optical fiber networks. These optical fiber networks are generally supported by an infrastructure of individual optical fibers, fiber bundles, or other optically conductive transmission mediums. Optical fibers, which are the most common transmission medium for optical signals, generally include an outer shell or body portion that supports an inner optically conductive core portion. The optically conductive core generally has a diameter of approximately 8 micrometers. The terminating ends of the individual optical fibers are received by various optical devices, such as an optical waveguide, for example. An optical waveguide may include at least one optical signal input, an inner optical core portion that is conductive for optical signals and in optical communication with the optical signal input, and at least one optical signal output that is in optical communication with the optical core. These waveguides operate to receive an optical signal from a first optical signal source at an optical input, transmit the optical signal through the core portion of the waveguide, and disseminate the optical signal to another optical device or another optical transmission medium at an output of the waveguide. This process is ideally conducted with minimal loss or distortion to the optical signal as it travels through the waveguide.
Optical waveguides exist at multiple levels. For example, an optical waveguide may be used in larger devices, such as a router for an optical network. Optical waveguides may also be used in devices as small as ICs. Optical waveguides are used at the IC level to communicate optical signals between various IC components. U.S. Pat. No. 5,464,860 to Fujimoto describes a conventional IC waveguide and a method for manufacturing such, as illustrated in FIG. 1. The waveguide of Fujimoto is formed by depositing a cladding layer 101 on a substrate 100, and then depositing a metal layer 102 over the cladding layer 101. A trench 106 having a rectangular shape is then anisotropically etched into the middle of the cladding layer 101 through the metal layer 102. The trench 106 is then filled with an active waveguide polymer 103. The polymer layer 103 in the rectangular trench 106 is then etched back to a level below the metal layer 102 and the trench 106 is backfilled with an optically non-conductive buffer layer 104, which operates to optically isolate the polymer layer 103 in the rectangular trench 106.
However, conventional optical fiber cores are circular, and therefore, an inherent mismatch exists between the circular fiber core and the rectangular core of conventional IC waveguides. This mismatch represents a potential loss and/or degradation region for optical signals traveling from a fiber into a waveguide. Another problem with conventional IC optical waveguides is that the core is generally sized to approximate the core dimension of standard optical fibers, which is generally 8 micrometers. This poses a substantial problem, as the current trend is to manufacture high refractive index devices having substantially smaller core dimensions, in the range of between about 8 micrometers and about 2 micrometers. High refractive index cores allow the design of OIC""s to be smaller as well as enable low-loss integration of silicon and class III/IV-based devices, such as lasers, amplifiers, detectors, and other devices into hybrid circuits. This presents a problem, as it is difficult to couple a standard 8 micrometer optical fiber to a device core that has a smaller dimension, for example xc2xc that of the optical fiber size, or about 2 micrometers, without incurring substantial signal loss or degradation.
Therefore, there exists a need for a method for manufacturing an IC optical waveguide that eliminates coupling mismatch loss and/or signal degradation. Further, there exists a need for an IC optical waveguide capable of coupling to optical sources having a core dimension that is substantially larger than the core dimension of the optical waveguide.
Embodiments of the invention generally provide a method for manufacturing an IC optical waveguide. The method includes depositing a cladding material on a first substrate, forming a trench in the cladding material on the first substrate, and filling the trench with a optically conductive core material. The upper surface of the cladding material and the optically conductive core material are then planarized to produce a substantially planar surface. The method further includes depositing a cladding material on a second substrate, forming a mirror image trench into the cladding material on the second substrate, and filling the mirror image trench with the optically conductive core material. The upper surface of the second cladding layer and the core material therein is then planarized. Thereafter, the first substrate is affixed to the second substrate such that the trench and the mirror image trench are in abutment and form a substantially circular optical core.
Embodiments of the invention also provide a method for forming a substantially circular optical channel in a waveguide. The method includes depositing a cladding layer on a substrate, etching a first trench in the cladding layer, the first trench having a substantially semi-circular cross section, and etching a mirror trench in the cladding layer, the mirror trench also having a substantially semi-circular cross section. The first trench and the mirror trench are filled with an optically conductive core material, and the upper surface of the cladding layer and an area over the first trench and the mirror trench is planarized. Thereafter, the mirror trench is folded onto the first trench and affixed thereto to form a substantially circular optical core surrounded by a continuous cladding layer.
Embodiments of the invention further provide an optical waveguide having a circular optical core. The waveguide includes a bottom portion and a top portion that are affixed together to form the waveguide. The bottom portion includes a bottom substrate, a first dielectric cladding layer is deposited on the bottom substrate and has a substantially planar first outer surface, a semi-circular trench is formed in the first cladding layer, and an optically conductive core material is concentrically positioned in the semi-circular trench and having a first surface that is coplanar with the first outer surface. The top portion includes a top substrate, a second dielectric cladding layer is deposited on the top substrate and has a substantially planar second outer surface, a semi-circular mirror image trench is formed in the cladding layer, and an optically conductive core material is concentrically positioned in the semi-circular mirror image trench and has a second surface that is coplanar with the second outer surface. The top and bottom portions are affixed together through a lamination or epoxy process to form the optical waveguide having a substantially circular optical core.
Embodiments of the invention further provide for a tapered transition between a waveguide made of xe2x80x9clowxe2x80x9d refractive index material such as a quartz optical fiber and an OIC waveguide made of a higher refractive index material such as silicon, silicon nitride, or silicon oxy-nitride. By appropriately designing the mask used to create the photoresist pattern for wet etching the trenches of the respective waveguide halves, it is possible to create a tapered expansion in the xe2x80x9cwidthxe2x80x9d dimension of the trench, i.e., along the longitudinal axis of the trench. The resulting effect, once the two waveguide halves are assembled, is a waveguide that is tapered as it approaches the edge of the substrate. This tapered feature will provide a lower insertion loss for a standard quartz fiber transition to a waveguide on the substrate, when the waveguide is made of a material that has a refractive index that is higher than quartz.
Embodiments of the invention further provide an improved tapered transition between a waveguide made of xe2x80x9clowxe2x80x9d refractive index material such as a quartz optical fiber and an OIC waveguide made of a higher refractive index material. This can be accomplished by taking advantage of a phenomenon related to wet etching of planar substrates that is known as an xe2x80x9cedge effectxe2x80x9d. By adjusting the etching bath chemistry, temperature, and circulation appropriately, it is possible to enhance the wet etch rate of the film near the edges of a substrate relative to the inner regions. By properly optimizing the bath conditions described, it is possible to achieve a deeper trench, as well as a wider trench near the edge of the substrate. The resulting effect, once the two waveguide halves are assembled, is a tapered xe2x80x9chornxe2x80x9d shape to the waveguide as it approaches the edge of the substrate. This tapered feature provides a lower insertion loss for a standard quartz fiber transition to a waveguide on the substrate, when the waveguide is made of a material that has a refractive index that is higher than quartz.